A carbon dioxide gas laser oscillation apparatus of a high-speed axial flow type, a typical gas laser oscillation apparatus, can produce high output and high-quality laser beams with a compact design, so that it has been widely utilized for laser work, such as cutting of metallic and nonmetallic materials and welding of metallic materials and the like. In particular, this apparatus is rapidly developing as a CNC laser work machine that is coupled with a CNC (numerical control device) and used in the field of high-speed, high-accuracy cutting work for complicated shapes.
In the carbon dioxide gas laser oscillation apparatus, about 20% of injected electrical energy is converted into laser beams, and the remainder is consumed in heating of a laser gas. On the other hand, the laser oscillation gain inevitably lowers as the temperature of the laser gas increases. In order to enhance the oscillation efficiency, therefore, it is necessary to cool the laser gas compulsorily, thereby making the laser gas temperature as low as possible. Accordingly, in the carbon dioxide gas laser apparatus of the high-speed axial flow type, a gas laser blower is used to circulate the laser gas compulsorily in the apparatus and feed it into the cooler.
FIG. 6 shows a vertical gas laser blower of an oil circulation type, as an example of this gas laser blower. In a gas laser blower 100, a shaft 2 that fixes an impeller 1 is arranged vertically. When the impeller 1 rotates, a laser gas is sucked in from a cooler (not shown) on the upstream side (or above the shaft) toward the impeller 1, passing in the direction of the shaft 2 (direction of arrow 9) along an upstream-side circulation path U. Then, the laser gas is caused to be discharged in a direction (direction of arrow 10) perpendicular to the direction of axis of the shaft by the impeller 1, and flows into a cooler (not shown) on the downstream side through a downstream-side circulation path D.
After the impeller 1 is fitted on the upper end portion of the shaft 2, it is fixedly coupled to the shaft 2 by fastening the upper end of the shaft 2 by means of a nut 7. The impeller 1 fixed to the shaft 2 is covered by a circulation-path-side casing 8 that defines the laser gas circulation paths U and D.
A rotor 3 is fixed to the axial central portion of the shaft 2 by shrink fitting. The rotor 3 is covered by a motor-side casing 12. A stator 4 is fixed to that part of the motor-side casing 12 which faces the rotor 3. The rotor 3 and the stator 4 constitutes a high-frequency motor 30. As a result, the impeller 1 is rotated at a high speed of tens of thousands of rpm.
The shaft 2 is supported on the apparatus body by means of a pair of bearings 5 and 6. The bearing 5 on the upper side is provided in a communication hole 121a that is formed in a partition wall 121 between the motor-side casing 12 and the circulation-path-side casing 8. The bearing 6 on the lower side is mounted on a shaft supporting portion 20 which will be mentioned later. Normally, angular ball bearings, which can stand high-speed rotation, are used as these bearings 5 and 6, individually.
An oil passage 13 is provided in the shaft 2, extending in the axial direction thereof. A plurality of oil outlets 131 with a small bore that connect the oil passage 13 and the outside are formed in the vicinity of such part of the shaft 2 as is supported by the bearing 5.
On the lower end side of the shaft 2, an oil suction head 17, having its outer periphery tapered toward the distal end (downward), is provided integrally with the shaft 2. This oil suction head 17 is provided with an oil passage 13 that communicates with the oil passage 13 in the shaft 2, and an oil inlet 130 of the oil passage 13 is formed at the lower end of the shaft 2.
Around the lower end portion of the shaft 2 or right under the motor 30, an oil reservoir 18 is fixed to the apparatus body. Around the oil suction head 17, moreover, the cylindrical shaft supporting portion 20 is fixed integrally to the oil reservoir 18. A bottom opening (not shown) of this oil reservoir 18 is connected to one end of an oil passage 180, and the other end of the oil passage 180 communicates with an aperture 201 in the center of the bottom portion of the shaft supporting portion 20. Thus, oil in the oil reservoir 18 flows into the cylindrical shaft supporting portion 20 via the oil passage 180 and the aperture 201. The oil injected into the shaft supporting portion 20 further gets into the oil passage 13 through the oil inlet 130 at the lower end of the oil suction head 17. As a result, oil in the oil reservoir 18, oil in the shaft supporting portion 20, and oil in the oil passage 13 are all kept on the same level when the impeller 1 is stationary.
A cylindrical bearing sleeve 21 is fitted in the cylindrical shaft supporting portion 20 for axial sliding motion. This bearing sleeve 21 includes a projecting portion 210 that projects for a given length toward the axis. The upper end of the projecting portion 210 is in contact with the outer ring of the bearing 6, and the lower end in contact with the free end of a spring 16. The proximal end of the spring 16 is fixed to the bottom of the shaft supporting portion 20. Thus, the elasticity of the spring 16 first acts as a force to push up the bearing sleeve 21, and in consequence, this force is transmitted to the bearing 6, shaft 2, and bearing 5 in succession. As a result, the bearings 5 and 6 that support the shaft 2 are kept under a pre-load applied by the spring 16.
An oil return passage 15 is provided extending along the axial direction of the shaft 2 in such part of the motor-side casing 12 as surrounds the stator 4 that constitutes the motor 30. Further, a cooling water passage 19 is provided extending along the oil return passage 15 in such part of the motor-side casing 12 as surrounds the region for the oil return passage 15.
The oil in the oil passage 13 in the oil suction head 17 is pressed against the inner wall surface of the oil passage 13 by means of a centrifugal force that is produced as the shaft 2 fitted with the impeller 1 rotates. As this is done, the oil is subjected to a component force in the direction to push up the oil along the inner wall surface. As a result, the oil is sucked up rapidly, and is discharged from the oil outlets 131 through the oil passage 13 in the shaft 2. Some of the discharged oil is supplied to the bearing 5 and used to lubricate the bearing 5. The discharged oil returns to the oil reservoir 18 through the oil return passage 15. As it passes through the oil return passage 15, the oil is cooled by cooling water in the cooling water passage 19. The oil returned to the oil reservoir 18 passes through the oil passage 180, communicates with the bottom opening of the oil reservoir 18, and flows into the cylindrical shaft supporting portion 20. Then, the oil introduced into the shaft supporting portion 20 further gets into the oil passage 13 through the oil inlet 130 at the lower end of the oil suction head 17. Thus, the oil continually circulates while the shaft 2 is rotating.
In the motor-side casing 12, a lubricant supplied to the bearing 6 evaporates, and impurity gas is generated from the drive motor 30. The resulting vapor and gas may possibly get into the circulation-path-side casing 8 through the bearing 5 and the communication hole 121a formed in the partition wall 121 to which the bearing 5 is attached, and be mixed in the laser gas flowing through the circulation paths.
More specifically, the bearings 5 and 6 support the shaft 2 which is rotating at high speed, so that the lubricant to be supplied to these bearings 5 and 6 has to be particularly highly lubricative. Therefore, those lubricants with extremely low vapor pressures which are used in conventional high-vacuum apparatuses cannot be used for the purpose on account of their low lubricating properties. As a result, the lubricant selected to be supplied to the bearings 5 and 6 is easily reduced to vapor despite their low vapor pressures.
Since the shaft 2 that rotates at high speed penetrates the partition wall 121 so as to be situated both in the motor-side casing 12 and in the circulation-path-side casing 8, on the other hand, the motor-side casing 12 and the circulation-path-side casing 8 cannot be isolated entirely from each other. Thus, the vapor from the highly lubricative lubricant itself, supplied to the bearings 5 and 6, and the gas generated from the motor 30 flow into the circulation-path-side casing 8 through the communication hole 121a in the partition wall 121, and gets into the laser gas circulation paths. In some extreme cases, the lubricant itself used to get into the circulation-path-side casing 8 through a narrow gap in the partition wall 121.
As a result, the laser gas, being contaminated with the aforesaid gas and vapor, entails reduction in the laser output. Further, the substances introduced from the motor-side casing 12 into the laser gas circulation paths, being deposited on an output coupler mirror, total reflector, and other optical components that constitute an optical resonator, cause reduction in the laser output, breakage of the optical components, etc. These problems are not peculiar to the gas laser blower of an oil lubrication type, but are also aroused in the cases of grease lubrication and other lubrication systems.
Therefore, to cope with these problems, the laser gas is extracted from the laser gas circulation paths and the quantity of replenishment is increased. However these conventional countermeasures would entail an increase in the running cost of the laser oscillation apparatus.
Further, it is devised to minimize the size of the communication hole 121a for bearing attachment provided in the partition wall 121 that divides the motor-side casing 12 and the circulation-path-side casing 8. But this countermeasure would cause problems such that the machining accuracy is limited and the costs are increased for nothing, so that its effect is unsatisfactory.
In one case, moreover, the shaft 2 is provided with a disk-shaped oil thrower in order to prevent the lubricant from getting directly into the circulation-path-side casing. Also in this case, however, the through hole cannot be narrowed due to the limited machining accuracy, and there is hardly any effect on the interception of the vapor from the lubricant and the gas generated from the motor 30.
In another case, furthermore, sealing means, such as a contact-type mechanical seal, is used for sealing between the motor-side casing 12 and the circulation-path-side casing 8. In this case, however, the reliability is very poor and the required life performance cannot be enjoyed, due to wear of sliding surfaces and frictional heat therefrom, so that this countermeasure is not practicable.